Dual gate tunable oscillator

ABSTRACT

A dual gate field effect transistor (FET) is configured as a self-buffering local oscillator of a tuner by arranging the FET in a cascode configuration in which the first gate electrode is coupled to the source electrode through an oscillation conditioning network and also to a frequency determining network, the second gate electrode is coupled to signal ground through a negligible impedance and the drain electrode is utilized as the output of the oscillator.

RELATED APPLICATIONS

U.S. application Ser. No. 935,438 entitled "Wide Range Oscillator" andU.S. application Ser. No. 935,439 entitled "FET Tuner" concurrentlyfiled in the name of the same inventor are related applications.

FIELD OF THE INVENTION

The present invention is in the field of tunable oscillators that may beemployed, e.g., as the local oscillator of a tuning system.

BACKGROUND OF THE INVENTION

Tuning systems for radio and television receivers typically include atunable RF stage for selecting the RF signal corresponding to a desiredstation or channel from a plurality of received RF signals, a tunablelocal oscillator for generating a local oscillator signal having afrequency corresponding to the desired station or channel and a mixerfor heterodyning the selected RF signal with the local oscillator signalto produce an IF signal corresponding to the RF signal.

The local oscillator should be capable of generating a local oscillatorsignal of sufficient amplitude to reliably drive the mixer and providean output impedance compatible with the input impedance of the mixer forefficient power transfer. In addition, the local oscillator should bearranged so that the mixer does not significantly interfere with theoperation of the local oscillator. While it is possible to employ aseparate buffer amplifier in conjunction with the local oscillator toprovide these desired features, it is not economical to do so.

SUMMARY OF THE INVENTION

In accordance with the present invention, a dual gate field effecttransistor (FET) is configured as a self-buffering tunable oscillator.Specifically, the FET is arranged in a cascode arrangement in which thefirst gate electrode is coupled to the source electrode through anetwork for establishing the conditions for oscillation and also to atuned circuit including a varactor diode and an inductance element fordetermining the particular frequency of oscillation, the second gateelectrode is coupled to a reference potential and the output of theoscillator is taken at the drain electrode.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram showing a tuner of a television receiverembodying the present invention;

FIG. 1a is a schematic diagram of an equivalent circuit useful inunderstanding an aspect of the local oscillator shown in FIG. 1;

FIG. 1b is a schematic diagram of a modification to the local oscillatorshown in FIG. 1;

FIG. 1c is a schematic diagram of an equivalent circuit useful inunderstanding the modification shown in FIG. 1b; and

FIG. 2 is a schematic diagram showing in detail the RF stage of thetuner shown in block form in FIG. 1.

In the figures, various exemplary element values are identified inparenthesis. Unless otherwise indicated, resistance values are in ohms,the capacitance values are in picofarads and the inductance values arein nanohenries. Further with regard to the exemplary values, Krepresents 1000, M represents 1,000,000 and μ (micro) represents0.000001.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the VHF section of a tuner of a television receiver fortuning VHF broadcast and VHF cable channels. RF signals provided by asource (not shown) such as an antenna or cable distribution network arecoupled via an RF input 1 to a tunable RF stage 3. RF stage 3 selectsthe RF signal corresponding to a desired channel in response to themagnitude of a tuning voltage (TV). The selected RF signal is coupled toa mixer 5 where it is heterodyned with a local oscillator signalgenerated by a local oscillator 7 having its frequency of oscillationcontrolled in response to the magnitude of the tuning voltage to producean IF signal corresponding to the selected RF signal.

A tuning control unit 9 generates the tuning control voltage. Tuningcontrol unit 9 also generates bandswitching voltages (BS1 and BS2) forselecting inductors to be included in frequency selective tuned circuitsof RF stage 3 and local oscillator 7 in accordance with the tuning bandof the desired channel. By way of example, tuning control unit 9 mayinclude a phase-locked loop (PLL) type of frequency synthesizer forconverting binary coded representations of the channel number of thedesired channel to a tuning voltage of the appropriate magnitude and alogic network for generating the appropriate bandswitching voltage inresponse to the binary coded representations of the channel number.

Bandswitching voltages BS1 and BS2 can have either a low level, e.g.,-12 volts, or a high level, e.g., +12 volts. The tuning bands and therespective levels of bandswitching voltages BS1 and BS2 are indicated inthe following table.

    ______________________________________                                                                 RF        LO                                                                  FREQUENCY FREQUENCY                                                CHANNEL    RANGE     RANGE                                      BS1   BS2     NOS.       (MHz)     (MHz)                                      ______________________________________                                        LOW   LOW     2 thru 6   55-88     101-129                                    HIGH  LOW     A-5 thru 13                                                                               91-216   137-257                                    HIGH  HIGH    J thru     217-468   263-509                                                  W + 28                                                          ______________________________________                                    

Local oscillator 7 comprises an amplifier 100 including a dual gate Nchannel metal oxide semiconductor (MOS) field effect transistor (FET)101 having a first gate electrode (G1), a second gate electrode (G2), anN-type conduction channel connected at one end to a source electrode (S)and at the other end to a drain electrode (D). The voltages at the gateelectrodes determine the degree of conduction of the conduction channel.A source of positive supply voltage (B+), e.g., +12 volts, is providedby tuner control unit 9 when a VHF channel is selected. The positivesupply voltage is filtered by a lowpass filter including a resistor 103and a capacitor 105. The power supply return path is connected to signalground. A voltage divider network including resistors 107, 109, 111 and113 connected to the gate electrodes biases FET 101 to operate as linearamplifier. Resistor 109 serves to inhibit unwanted parasiticoscillations of FET 101.

In local oscillator 7, amplifier 100 is configured as a cascodeamplifier by utilizing the first gate electrode (G1) of FET 101 as theinput, effectively connecting the second gate electrode (G2) to signalground through a bypass capacitor 115 (noting that resistor 109 has avery small value), coupling the source electrode (S) to signal groundthrough a resistor 117, and utilizing the drain electrode (D) as theoutput of the amplifier. The drain electrode (D) is coupled to the B+power supply conductor through a load resistor 119 and to mixer 5through a large valued DC blocking capacitor 121. A ferrite bead 123 isprovided on the conductor between load resistor 119 and the B+ conductoras an inductive AC blocking filter element. The configuration of FET 101may be thought of as a cascode amplifier because the first gateelectrode (G1), the source electrode (S) and the lower end of theconduction channel are configured as a common source amplifier and theupper end of the conduction channel, the second gate electrode (G2) andthe drain electrode (D) are configured as a common gate amplifier.

A circuit 200 for conditioning amplifier 100 to oscillate is coupledbetween the first gate electrode (G1) and the source electrode (S). Aseries-tuned circuit 300 responsive to the tuning voltage (TV) fordetermining the particular frequency of oscillation is coupled betweenthe first gate electrode (G1) and signal ground.

Oscillator 7 is conditioned to oscillate in the following manner. Ingeneral, an amplifier will oscillate if two conditions are met: (1)there is zero phase shift around a loop including a path from the inputto the output of the amplifier and a path from the output to the input;and (2) the gain around the loop is greater than unity. In the case ofoscillator 7, the portion of FET amplifier 100 including the first gateelectrode (G1), the source electrode (S) and the lower end of theconduction channel is conditioned to oscillate. While this portion is acommon source amplifier with regard to the cascode amplifierconfiguration of FET 101, it is a common drain or source followamplifier, with an input at the first gate electrode (G1) and the outputat the source electrode (S), with regard to the oscillationconfiguration. Oscillation conditioning network 200, connected betweenthe output of the common drain amplifier configuration at the sourceelectrode (S) and the input at the first gate electrode (G1), includes acapacitor 201 connected in shunt with resistor 117 between the sourceelectrode (S) and signal ground and a capacitor 203 connected betweenthe source electrode (S) and the first gate electrode (G1). As will beappreciated, this configuration is of the Colpitts type.

With respect to the phase shift requirement for oscillation, there issubstantially no phase shift between the input (G1) and the output (S)and there is a phase lag contributed by capacitor 201 and a compensatingphase lead contributed by capacitor 203 between the output (S) and theinput (G1). With respect to the gain requirement for oscillation, thereis a voltage gain of slightly less than one due to source followeroperation between the input (G1) and the output (S) but a voltageincrease ("step-up") due to capacitors 201 and 203 between the output(S) and the input (G1). As a result, the conditions for oscillation aremet and the source follower configuration will oscillate at thefrequency determined by tuned circuit 300. The current through theresistor 117 connected to the source electrode (S) and through theconduction channel varies with the oscillation and the voltage acrossload resistor 119 connected to the drain electrode (D) variesaccordingly.

The cascode amplifier configuration of FET 101 is advantageous inseveral respects. The common gate amplifier portion provided bybypassing the second gate electrode (G2) to signal ground substantiallyisolates the oscillatory portion from mixer 5 while additionallyenabling mixer 5 to be driven at appropriate signal and impedance levelswithout the need for a separate buffer amplifier device. Due to thecommon gate amplifier portion, a virtual ground is effectively presentedto the output of the common source amplifier portion so that impedancevariations exhibited by the mixer, due, e.g., to amplitude variations inthe RF signal coupled to it from RF stage 3, do not substantially effecteither the frequency of oscillation or the conditions for establishingoscillation. Moreover, because of the isolation, the drive requirementsof mixer 5 do not have to be compromised to satisfy the conditionsnecessary for oscillation.

Another advantageous feature of FET local oscillator 7 is realized whenit is employed with an FET RF stage. Many tuners used in televisionreceivers employ dual gate FET RF stages because they produce relativelylow distortion and have relatively high impedances compared with bipolartransistor RF stages. In addition, the second gate electrode provides aconvenient means for applying an automatic gain control (AGC) voltage. Adual gate FET RF stage suitable for use as RF stage 3 is shown in FIG. 2and will be explained in detail below. Briefly, the RF stage shown inFIG. 2 includes an amplifier 400 including a dual gate N MOS FET 401configured, like FET 101 of local oscillator 7, as a cascode amplifierwith the input at the first gate electrode (G1), the second gateelectrode G2 effectively connected to signal ground through a bypasscapacitor, the source electrode (S) coupled through a resistor to signalground, and the output derived at the drain electrode (D). RF input 1 iscoupled to the input (G1) of FET amplifier 400 through a series tunedcircuit 500 responsive to the tuning voltage (TV). The output of FETamplifier 400 is coupled through a doubly tuned filter 600, comprisingtwo inductively coupled series-tuned circuits 601 and 603 eachresponsive to the tuning voltage (TV), to another dual gate FETamplifier 700 also configured in cascode configuration. The output ofdual gate FET amplifier 700 is coupled to mixer 5. Since RF stage 3 andlocal oscillator 7 have amplifiers of the same device type andconfiguration and have similar tuning configurations, it has been foundthat the ability to track one another in frequency in response to thetuning voltage is improved compared with a conventional arrangement inwhich the RF amplifier is of the dual gate FET type and the localoscillator is of the bipolar type.

Returning now to FIG. 1, tuned circuit 300 will be described in detail.As earlier noted, tuned circuit 300 is a series-tuned circuit. Tunedcircuit 300 includes inductors 301, 303 and 305, a varactor diode 307connected in series with a DC blocking capacitor 309 between the input(G1) of amplifier 100 and signal ground. Inductor 305 is coupled inseries between varactor diode 307 and the input (G1) of amplifier 100.This has been found to be a beneficial configuration since the inductor305 tends to isolate varactor 305 from stray capacitances exhibited atthe input of amplifier 100. Bandswitching diodes 313 and 311 andassociated bypass capacitors 315 and 317, respectively, selectivelybypass the circuit point intermediate inductors 301 and 303 and thecircuit point intermediate inductors 303 and 305, respectively, tosignal ground in accordance with the levels of bandwitching voltages BS1and BS2. Bandswitching voltages BS1 and BS2 are coupled to bandswitchingdiodes 313 and 315 through respective high valued isolation resistors317 and 319, respectively. The tuning voltage (TV) is filtered by alowpass filter including a resistor 325 and a capacitor 327 and coupledto the cathode of varactor diode 307 through isolation resistors 321 and324 and inductor 305.

An oscillation range extending circuit 205 associated with oscillationconditioning network 200 includes a capacitor 207 and a varactor diode209 directly connected in series between the input (G1) of amplifier 100and signal ground without any intervening elements that would presentsignificant impedance in the frequency range of interest. Capacitor 207has a capacitance value selected so that it appreciably effects thecombined capacitance of capacitor 207 and varactor diode 209. Inpractice, the specific value of capacitor 207 can be selected to controlthe range extension and the tracking of local oscillator 7 with RF stage3. The tuning voltage (TV) is coupled to the cathode of varactor diode331 through isolation resistor 321. Varactor diodes 307 and 209 arepoled with respect to the tuning voltage so that the capacitances theyexhibit change in the same sense in response to changes in the magnitudeof the tuning voltage. Range-extending circuit 205 extends theoscillation range of oscillator 7 in the following manner.

The equivalent circuit exhibited by amplifier 100 at its input (G1) inthe range of oscillation is shown in FIG. 1a and includes an equivalentcapacitance element (C_(eq)) and a negative resistance element (-R_(eq))connected in series between the first gate electrode (G1) and signalground. The negative resistance element (-R_(eq)) is related to the gainexhibited by the oscillation portion of amplifier 100. The equivalentcircuit exhibited by the series-tuned circuit 300 including inductors301, 303 and 305, varactor diode 307 and DC blocking capacitor 309 atthe input (G1) of amplifier 100 includes a variable capacitance element(C_(T)), a resistance element (R_(T)) and an inductance element (L_(T))connected in series between the first gate electrode (G1) and signalground. Since DC blocking capacitor 309 has negligible impedance in thefrequency range of interest, the variable capacitance element (C_(T))essentially exhibits the capacitance of varactor diode 307. Theresistance R_(T) corresponds to the loss associated with the tunedcircuit, which is primarily associated with varactor diode 307. Tosustain oscillation throughout the range of interest (101-509 MHz), themagnitude (R_(eq)) of the negative resistance element (-R_(eq))associated with amplifier 100 must be greater than the magnitude of theresistance element (R_(T)) associated with tuned circuit 300. Theparticular frequency of oscillation is inversely related to the squareroot of L_(T) C, where C is the combined capacitance of C_(T) andC_(eq). The combined capacitance of C_(T) and C_(eq) is given by C_(T)C_(eq) /C_(T) +C_(eq). For a wide tuning range, C_(eq) should be aslarge as possible with respect to the largest value of C_(T)(corresponding to the lowest frequency of oscillation) so that C canundergo substantially the full range of change of varactor diode 307(C_(T)).

Adding a fixed capacitor in shunt with the input of amplifier 100between the first gate electrode (G1) and signal ground increases thevalue of C_(eq) and therefore tends to extend the tuning range at lowfrequencies. However, the addition of a fixed shunt capacitor decreasesR_(eq) and therefore tends to prevent oscillation, especially at highfrequencies. Range extension circuit 205 connected in shunt with theinput (G1) of amplifier 100 provides a variable capcitance whichincreases as the tuning voltage (frequency) decreases and whichdecreases as the tuning voltage (frequency) increases. As a result,C_(eq) is the largest when C_(T) is the largest (i.e., at lowfrequencies) but a value R_(eq) sufficiently large to sustainoscillations is still provided at high frequencies.

The direct connection of range extending circuit 205 between the input(G1) of amplifier 100 and signal ground, rather than through an elementhaving significant impedance in the frequency range of interest, ensuresthat it will have an appreciable effect on the input capacitance(C_(eq)) of amplifier 100.

With regard to range extension network 205, it is noted that while adual gate FET has the advantages described above, its gain (andtherefore R_(eq)) is lower than that of a bipolar transistor configuredin comparible fashion as a common collector Colpitts type oscillatorwith its base electrode coupled to a tuned circuit, its emitterelectrode coupled to signal ground through an impedance and itscollector electrode serving as the output electrode. Therefore, whilerange extension circuit 205 may be utilized to extend the tuning rangeof a Colpitts type bipolar transistor oscillator, its advantages areeven more significant when employed with an FET oscillator as shown inFIG. 1.

It is known to employ a parallel tuned circuit in place of a seriestuned circuit as is shown in simplified form (without biasing elements)in FIG. 1b. However, it was found that when a parallel tuned circuit wasutilized in place of series tuned circuit 300 it was more difficult toobtain the wide tuning range required, even when a range extendingcircuit was utilized as is shown in FIG. 1b. This can be explained asfollows with respect to the equivalent circuit shown in FIG. 1c. InFIGS. 1b and 1c, elements corresponding to the same elements in FIGS. 1and 1a, respectively, are identified by the same reference designations.The primed (') designations correspond to the modification of replacingthe series tuned circuit with a parallel tuned circuit.

With respect to FIG. 1c, the frequency of oscillation is inverselyrelated to the square root of L_(T) C', where C' is the combinedcapacitance of C' _(T) and C_(eq). In this case, the combinedcapacitance C' is given by C'_(T) +C_(eq). For a wide tuning rangeC_(eq) should be small with respect to the lowest value of C'_(T)(corresponding to the highest frequency of oscillation) so that C' canundergo substantially the full range of change of C'_(T). The value ofC_(eq) can be lowered by connecting a low valued capacitor in seriesbetween the parallel tuned circuit and the input of the amplifier.However, the effective loss of the parallel tuned circuit increases asthe square of the ratio C_(T) /C_(eq) and therefore at the high valuesof C_(T) (corresponding to the low frequency end of the tuning range),the loss of the parallel tuned circuit may overcome the gain (related to-R_(eq)) required for oscillation.

The addition of a varactor diode connected in series between theparallel tuned circuit and the input of the amplifier and poled so thatits capacitance varies in the same sense as the varactor diode of thetuned circuit as shown in FIG. 1b tends to extend the tuning range byproviding compromise between a relatively low value of C_(eq) at highfrequencies and a relatively low effective loss at low frequencies.However, it was found that since the loss of the parallel tuned circuitvaries with the square of the ratio C_(T) /C_(eq) at the lower frequencyend of the required tuning range, oscillation was not always reliablewhen an FET was utilized rather than a higher gain (higher R_(eq))bipolar transistor. Accordingly, the series tuned configuration shown inFIG. 1 is preferred for use with a FET.

As earlier noted, tuning control unit 9 may comprise a phase lockedloop. The reliable oscillation of oscillator 7 at low frequencies isparticularly important when a phase locked loop type of tuning controlsystem is employed. Phase locked loop tuning control systems usuallyemploy a frequency divider known as a "prescaler" for dividing the veryhigh frequency of the local oscillator signal before it is furtherdivided by a programmable frequency divider according to the channelnumber and thereafter compared with a reference frequency to generatethe tuning voltage. Some prescalers have shown an undesirable tendencyto oscillate and if the local oscillator does not oscillate reliably,the phase locked loop may respond to the oscillatory signal of theprescaler rather than to the local oscillator signal. Since thefrequency of oscillation of the prescaler tends to be high, the phaselocked loop causes the tuning voltage to decrease to attempt to decreasethe perceived frequency of oscillation of the local oscillator. Thistends to further hamper the ability of the local oscillator to oscillateand the phase locked loop is erroneously "locked" at the wrongfrequency. Therefore, range extension network 205 is particularlyadvantageous when a phase locked loop or other type of closed loopfrequency synthesis tuning control system such as a frequency lockedloop is employed.

Returning now to FIG. 2, as earlier noted, tuned circuit 500 associatedwith FET amplifier 400 of RF section 3, like tuned circuit 300associated with FET amplifier 100 of local oscillator 7, is a seriestuned circuit. Series tuned circuit 500 includes a plurality ofinductors 501, 503, 505, 507, 509 and 511 which are selectivelyconfigured in different series tuned circuits together with a varactordiode 513 (actually two varactor diodes connected in parallel) dependingon the tuning band. The particular series tuned circuit configuration isdetermined by bandswitching diodes 515, 517 and 519, the conduction ofwhich is controlled by the levels of bandswitching voltages BS1 and BS2.The RF input signal is coupled to the junction of inductors 503 and 505.Series tuned circuit 500 is coupled to the first gate electrode (G1) ofFET 401 through a coupling capacitor 521.

A varactor diode 523 is connected in shunt with the first gate electrode(G1) and is poled so that its capacitance changes in the same sense asvaractor diode 513 in response to changes of the magnitude of the tuningvoltage (TV). Varactor diode 523 serves to make the impedance presentedby series tuned circuit 500 and the impedance presented at the input(G1) of amplifier 400 more closely match for optimum power transferthroughout the tuning range than otherwise. The function of varactordiode 523 associated with amplifier 500 of RF section 3 is not the sameas the function of range extending varactor diode 209 associated withamplifier 100 of local oscillator 7. However, the two similarlyconnected diodes do tend to make the tuning configurations similar and,therefore, tend to benefit tracking between RF stage 3 and localoscillator 7.

It is noted that inductor 511 is connected in series between varactordiode 513 and the input (G1) of amplifier 500 and corresponds to thesimilar connection of inductor 305 in series between varactor diode 307and the input (G1) of amplifier 100 in local oscillator 7. Thissimilarity of tuning configuration also tends to benefit the trackingbetween RF stage 3 and local oscillator 7.

While the second gate of FET amplifier 401 is bypassed to ground withrespect to RF signals, an automatic gain control (AGC) voltage iscoupled to it from the IF section of the receiver to control the gain ofamplifier 400 as a function of signal strength.

As earlier noted, the output of FET amplifier 400 is coupled to theinput of FET amplifier 700 through doubly tuned filter 600 including twoinductively coupled series tuned circuits 601 and 603. Series tunedcircuits 601 and 603 include respective pluralities of inductors 605,607 and 609 and 611, 613 and 615 connected in series with respectivevaractor diodes 617 and 619. Tuned circuits 601 and 603 includerespective bandswitching diodes 621 and 623 and 625 and 627. Tunedcircuit 601 is coupled to the output (D) of FET amplifier 400 through acoupling capacitor 629. An impedance matching varactor diode 631 isconnected in shunt to the output (D) of FET amplifier 400 and has asimilar function to impedance matching diode 523 connected in shunt withthe input (G1) of amplifier 400. A corresponding impedance matchingdiode 633 is connected in shunt with the input of FET amplifier 700.Another varactor diode 635 is connected in series between inductor 611and the input of FET amplifier 700 and also serves as an impedancematching device. Inductor 609 is connected in series between the output(D) of amplifier 400 and varactor diode 617 and inductor 611 isconnected in series between the input of amplifier 700 and varactordiode 619. Like inductor 511 associated with RF amplifier 400 andinductor 305 associated with local oscillator 7, inductors 605 and 611serve to isolate the respective varactor diodes from stray capacitances.Here again, since tuned circuits 601 and 603 are configured in similarmanner to tuned circuit 300 of local oscillator 7 and are loaded insimilar fashion (noting that a relatively high impedance is exhibited atthe drain as well as the gate electrode of an FET), tracking between RFstage 3 and local oscillator 7 tends to be benefited.

While tracking is benefited by the similar tuning circuit and amplifierconfigurations of RF stage 3 and local oscillator 7, it has been founddesirable because of the relatively large tuning range in the presentembodiment to employ another enhancement to tracking. Specifically,referring back to FIG. 1, it is noted that a bandswitching diode 333 anda low value capacitor 335 are connected in series across varactor diode307 and inductor 305. Bandswitching voltage BS1 is applied to thecathode of bandswitching diode 333 through a network including a filtercapacitor 337 and an isolation resistor 339. The anode of bandswitchingdiode 333 is coupled to signal ground through inductors 301 and 303.Bandswitching diode 333 is rendered conductive in the lowest tuningrange when bandswitching voltage BS1 is at the low level (-12 volts).The additional capacitance has been found to aid tracking at the upperfrequency end of the lowest tuning range.

A type 1SV161 varactor diode commercially available from Toshiba andtype BF994 or 3SK137 FETs commercially available from Siemens andHitachi, respectively, are suitable for use in the circuitry shown inthe various figures.

While the present invention has been described in terms of the VHFsection of a tuner, it can also be utilized in the UHF section. For UHFapplication, capacitor 203 of the oscillation conditioning network 200of local oscillator 7 may comprise an internal capacitance element.These and other modifications are intended to be within the scope of theinvention defined by the following claims.

I claim:
 1. Apparatus including a tunable oscillator, comprising:a dualgate field effect transistor (FET) having first and second gateelectrodes, a source electrode and a drain electrode; impedance meansfor coupling said source electrode to a point of reference potential;oscillation conditioning means connected between said first gateelectrode and said source electrode for conditioning said FET tooscillate in a predetermined frequency range; frequency determiningmeans including a varactor diode which exhibits a capacitance whichchanges in response to changes in the magnitude of a tuning voltage andan inductance element coupled to said first gate electrode fordetermining the oscillation frequency at which said FET oscillates;bypass means for effectively connecting said second gate electrode tosaid point of reference potential through a negligible impedance in saidpredetermined frequency range; and utilization means coupled to saiddrain electrode for receiving an output signal at said oscillationfrequency.
 2. The apparatus defined in claim 1 wherein:said FET is ofthe metal oxide semiconductor (MOS) type.
 3. The apparatus recited inclaim 1 wherein:said utilization means comprises mixer means for mixingsaid output signal provided at said drain electrode with an RF signal toproduce an IF signal.
 4. The apparatus defined in claim 1 wherein:saidoscillation conditioning means comprises a first capacitance elementcoupled between the first gate electrode and the source electrode and asecond capacitance element coupled between said source electrode andsaid point of reference potential.
 5. The apparatus recited in claim 4wherein:said oscillation conditioning means includes range extendingmeans including a second varactor diode also responsive to said tuningvoltage for ensuring that said FET oscillates throughout saidpredetermined range.
 6. The apparatus recited in claim 1 wherein:saidinductance element and said first mentioned varactor diode of saidfrequency determining means are connected in series between said firstgate electrode and said point of reference potential; and said secondmentioned varactor diode of said range extending means is coupledbetween said first gate electrode and said point of reference potentialand is poled to exhibit a capacitance which changes in the same sense asthe capacitance of said first mentioned varactor diode changes inresponse to changes in the magnitude of said tuning voltage. 7.Apparatus comprising:a dual gate metal oxide semiconductor (MOS) fieldeffect transistor (FET) having first and second gate electrodes, asource electrode and a drain electrode; impedance means coupled betweensaid source electrode and a point of reference potential; oscillationconditioning means connected between said first gate electrode and saidsource electrode for conditioning said FET to oscillate in apredetermined range; frequency determining means including a varactordiode exhibiting a capacitance which changes in response to changes inthe magnitude of a tuning voltage and an inductor for determining theparticular oscillation frequency at which said FET oscillates; bypassmeans for effectively connecting said second gate electrode to saidpoint of reference potential through a negligible impedance in saidpredetermined range; RF means for providing an RF signal; and mixermeans coupled to said RF means to receive said RF signal and coupled tosaid drain electrode of said FET to receive a local oscillator signal atsaid oscillation frequency for combining said RF signal and said localoscillator signal to produce an IF signal.
 8. The apparatus recited inclaim 7 wherein:said oscillation means includes range extending meansincluding a second varactor diode also responsive to said tuning voltagefor ensuring that said FET oscillates throughout said predeterminedfrequency range.
 9. The apparatus recited in claim 8 wherein:saidinductance element and said first mentioned varactor diode of saidfrequency determining means are coupled in series between said firstgate electrode and said point of reference potential; and said secondvaractor diode of said range extending means is coupled between saidfirst gate electrode and said point of reference potential and is poledto exhibit a capacitance which changes in the same sense as thecapacitance of said first mentioned varactor diode changes in responseto changes in the magnitude of said tuning voltage.
 10. Apparatuscomprising:a dual gate metal oxide semiconductor (MOS) field effecttransistor (FET) having first and second gate electrodes, a sourceelectrode and a drain electrode; impedance means coupled between saidsource electrode and a point of reference potential; oscillationconditioning means connected between said first gate electrode and saidsource electrode for conditioning said FET to oscillate in apredetermined range; frequency determining means including an inductorand a varactor diode coupled to said first gate electrode and responsiveto a tuning voltage for determining the particular oscillation frequencyat which said FET oscillates; said oscillation conditioning meansincludes range extending means including a second varactor diode alsoresponsive to said tuning voltage for ensuring that said FET oscillatesthroughout said predetermined frequency range; bypass means foreffectively connecting said second gate electrode to said point ofreference potential through a negligible impedance in said predeterminedrange; RF means for providing an RF signal; and mixer means coupled tosaid RF means to receive said RF signal and coupled to said drainelectrode of said FET to receive a local oscillator signal at saidoscillation frequency for combining said RF signal and said localoscillator signal to produce an IF signal.
 11. The apparatus recited inclaim 8 wherein:said inductance element and said first varactor diode ofsaid frequency determining means are coupled in series between saidfirst gate electrode and said point of reference potential; and saidsecond varactor diode of said range extending means is coupled betweensaid first gate electrode and said point of reference potential and ispoled to exhibit a capacitance which changes in the same sense as thecapacitance of said first varactor diode changes in response to changesin the magnitude of said tuning voltage.